The present invention relates to a permanent magnet dynamoelectric rotating machine and to an electric vehicle equipped with the same. More particularly, the present invention relates to such a dynamoelectric rotating machine which has a plurality of permanent magnets fixedly arranged along the circumference of a rotor. As used herein, the term "dynamoelectric rotating machine" includes an electromechanical device for converting electrical energy into mechanical energy, and vice versa. Hereinafter, for simplicity such devices will be referred to as "electric machines".
Permanent magnet electric machine of this generic type are known. For example, JP-A No. 5-76146 discloses a high-torque, high-efficiency permanent magnet electric machine which has a rotor with embedded permanent magnets. An annular stator core is provided with a plurality of slots, with stator windings for three phases embedded therein. A substantially cylindrical rotor core is mounted on a shaft, and has a plurality of axial slots in its inner circumference, with permanent magnets--having a rectangular section--embedded therein and oriented so that adjacent permanent magnets generate magnetic fluxes of opposite polarities toward the outer circumference of the rotor. The rotor is supported for rotation in the annular stator, with a predetermined air gap between the inner circumference of the stator core and the outer circumference of the rotor.
Such an electric machine, which uses permanent magnets with a rectangular section, is able to operate efficiently because the field when weakens it is operating at a high rotating speed. Therefore, the permanent magnet electric machine can effectively be applied to uses which require operation at a high rotating speed, such as a drive motor for an electric vehicle.
This conventional permanent magnet electric machine, however, has the following problems in the waveform of an induced voltage.
When the rotor is driven by an external force, an induced voltage is generated in the electric machine, causing a current to flow through a power circuit or a control circuit. The effective (that is, rms) induced voltage which will be generated at a particular rotating speed can be determined beforehand, and the control circuit can be designed to withstand the effective induced voltage (or to suppress the effective induced voltage to protect the control circuit).
However, an actual induced voltage has a waveform produced by superposing several waveforms on a sinusoidal waveform. Since the effective (rms) value is the average value of the varying voltage represented by the waveform, the waveform representing the actual induced voltage necessarily has a peak which is more than 2 times the effective value. Therefore, the peak value must be nearly equal to 2 times the effective value to protect the control circuit designed so as to reliably withstand a voltage equal to 2 times the effective value.
The reduction of the magnetic flux density of magnetic fields created by the permanent magnets is a possible means for reducing the peak value. However, the reduction of the magnetic flux density necessarily entails a reduction of the driving torque of the permanent magnet electric machine.
In order to provide a solution to the foregoing problems, it is an object of the present invention to suppress the peak value of an induced voltage relative to an effective value of the induced voltage, without reducing the driving torque of a permanent magnet electric machine.
Another object of the present invention is to provide an electric vehicle having enhanced safety, capable of suppressing the peak value of an induced voltage generated by a permanent magnet electric machine when the electric vehicle is braked or when the same travels a downhill slope.